The present invention relates to a condensation nucleus counter and its use.
In view of the high demands of modern clean-room technology, the need to detect particles of a diameter of &lt;0.1 mm in gases is increasing constantly. The only method which allows the detection of particles already in the nanometer range consists of enlarging the particles by the condensation of steam in such a manner that they can be measured in another, for example, optical manner.
A typical apparatus of this type is the Model 3760 condensation nucleus counter of the firm Thermo System INC, Minneapolis, Minn. This nucleus counter comprises a humidifier zone as well as a condensation zone, the condensation zone being arranged at a right angle with respect to the humidifier zone. A duct, through which sample air is guided, leads through the humidifying zone and the condensation zone. The humidifying of the sample air in the condensation zone takes place by guiding the sample air over a humidifying bath containing the process liquid. In order to achieve a relatively high mass flow, the nucleus counter of the prior art provides a tube system which divides the mass flow into identical partial flows. The mass flow achieved in this manner amounts to 1.4 liters/min.
It is a disadvantage of the nucleus counter of the firm Thermo System that, despite expensive constructive measures, it can receive only a mass flow of 1.4 liters per minute. For measurements, as they are required today in modern clean-room technology, a mass flow of this type is too low. It is also a disadvantage that sealing problems occur, and a loss of particles is also possible.
From the German Patent Document DE-GM 73 21 827, a nucleus counter is known which has a humidifying zone consisting of a permeable material.
A disadvantage of the nucleus counter according to the German Patent Document No. DE-GM 73 21 827 is that, in the case of multiple measurements, i.e., when measurements are to be made at several measuring points simultaneously, in each case, individual apparatuses are required, that is, individual nucleus counters with all necessary peripheral equipment. This results in high expenditures with respect to equipment and cost.
It is therefore an object of the invention to provide a nucleus counter which permits the measuring of larger mass flows and the carrying-out of a measurement simultaneously at various points with relatively low expenditures with respect to equipment.
This and other objects are achieved by the present invention which provides a condensation nucleus counter comprising a humidifying zone having a first duct of a permeable material, through which sample air is guided, the first duct being surrounded by a hollow space that receives humidifying liquid. A condensation zone is arranged approximately perpendicularly with respect to the first duct, the condensation zone including a second duct which is surrounded by a hollow space which receives a coolant. A connection piece fluidly couples the condensation zone with the humidifying zone. The nucleus counter also has an optical detection system including a light source and a detector.
It is particularly advantageous for the duct of the humidifying zone to have a ring-shaped cross-section. However, the ring-shaped cross-section is only a preferred embodiment. Other shapes, such as elliptic or rectangular shapes, are just as suitable for the nucleus counter according to the invention.
It is advantageous for the permeable material to have a porous structure, such as sintered bodies, foams, nonwovens or the like. A sintered material with a wall thickness of from 3-15 mm is particularly preferred, in which case the construction with the wall thickness of from 3-8 mm is preferred. This duct made of the permeable material will then be used for the aerodynamic separation of the individual aerosol flows. This duct also dips into the process liquid which, as a result of capillary forces, penetrates through the porous material and thus provides a permanent feeding of liquid. The losses of the process liquid are compensated by a supply from the outside.
On the other hand, the porous material should be so fine-grained that, despite the hydrostatic excess pressure existing from the outside, the humidifier duct will not fill up with liquid. The size of the wall thickness therefore depends on the material and is generally between 2 and 15 mm. Preferably, it is between 3-8 mm. The geometrical dimensioning of the humidifying zone, in this case, should be such that a complete humidification of the aerosol flow is ensured. This will exist when the non-dimensional parameter A from D.times.T/a.sup.2 is in the range of approximately between 0.5 and 1.5. It is preferable for the parameter to reach the magnitude of 1. In this case, D is the diffusion coefficient of the process steam in the carrier gas; T is the dwell time of the aerosol volume in the humidifier; and a is a typical dimension of the humidifier transversely to the flow direction, thus, for example, in the case of a circular tube, the tube radius. The condensation zone is constructed in the same manner. The dimensioning of the condenser takes place analogously, the temperature conductivity of the carrier gas (K) now replacing the process steam diffusion coefficient, and the numerical value of parameter B being in the range of between 0.05 and 0.4, so that the condensation nuclei will have sufficient time for growing into sufficiently large droplets, and excessive steam will not condense on the wall. The value of B should preferably be between 0.1 and 0.3.
Preferably, the duct of the condensation zone has a special construction. It is advantageous in this case to use a tube with a circular cross-section, the tube tapering, following the transition area, starting from the connection piece to the transition area to the adapter. In the simplest case, this may take place for a pressing-together of a tube, such as a copper tube. The tube will then preferably have an elliptic cross-section. An optimal growth of the droplets is achieved by means of this construction of the condensation duct. It is also advantageous for the lens system to be connected with the duct of the condensation zone by an adapter which is constructed such that the grown particles can be supplied to the lens system without any losses. The adapter preferably has a conical structure. In order to ensure a constant mass flow, the sample gas is sucked through by means of a pump, a critical nozzle being connected in front of the pump.
Preferably, it is now also possible to combine several individual measuring ducts to form a nucleus counter unit. This is implemented by the fact that for all ducts of the humidifying zone as well as of the condensation zone, only a single process bath or a single cooling device is provided. The housing of the humidifying zone therefore surrounds not only a single measuring duct but all measuring ducts of a measuring unit. In the same manner, the housing of the condensation zone surrounds all ducts of the cooling zone. As a result of this advantageous development, only one heating bath and one cooling device are required for the whole measuring unit. Likewise, only one pump is necessary for the whole system. As a result, the operating and constructive expenditures are lowered drastically and multiple measurements are made possible at the same time.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.